The research plan outlined in this proposal deals with the regulation of 3-Hydroxy-3-methylglutaryl coenzyme A reductase (HMG-CoA reductase, EC 1.1.1.34), considered to be the rate limiting enzyme for sterol biosynthesis. The enzyme from rat liver will be purified to homogeneity by a slight modification of our earlier procedure and analyzed for homogeneity by procedures more rigorous than normally employed. Antiserum to purified HMG-CoA reductase as well as that already made to our original preparation which has been purified will be used to study the mechanisms of the end-product inhibition by cholesterol. Studies will be continued on the lipid-interactions of the purified HMG-CoA reductase by reconstitution into defined lipids containing fluorescent and electron spin resonance probes. Furthermore, a study will be initiated to investigate whether excess cholesterol induces structural modification of hepatic microsomal membranes, using spectroscopic methods. Finally, isolated intestinal villous and crypt cells will be used to study the effectiveness of bile acids and cholesterol in regulating cholesterogenesis.